During vehicle deceleration, low engine torque is commonly requested to maintain a good drivability with controlled deceleration. However, the lowest torque that the engine produce during this mode is limited by the misfire limit of the engine. Typically, the engine has to be operated above a threshold load to reduce low load misfires. One way to reduce the engine torque output under the minimum load constraint is to operate the engine with only some of the cylinders firing while the remaining cylinders pump air without injected fuel. However, in a system that is to maintain stoichiometric operation with a three-way catalyst (TWC), when only some cylinders are fired, the air from the non-firing cylinders makes the exhaust air-fuel ratio mixture lean and can also saturate the exhaust system with oxygen. Under these conditions, the TWC has a degraded NOx conversion efficiency and hence the NOx coming from the firing stoichiometric cylinders may not be reduced. This can cause large NOx emissions. As a result, such a control system results in only minimal use of cylinder deactivation.
One solution would be to utilize a NOx trap to treat the mixture of combusted and non-combusted gasses. However, this can add significant cost and may not be able to meet emission requirements for all engine applications.
Another method to overcome the above disadvantages is to utilize a computer readable storage medium having stored data representing instructions executable by a computer to control an internal combustion engine of a vehicle, said engine having at least a first and second group of cylinders, with a first emission control device coupled exclusively to said first group of cylinders and a second emission control device coupled to said second group of cylinders, said storage medium comprising:
instructions for determining a requested engine output;
instructions for operating both the first and second group of cylinders near stoichiometry in first region and adjusting at least airflow to provide said requested engine output; and
instructions for operating said first group near stoichiometry and second group without injected fuel in second region where said engine output request is lower than in said first region, adjusting at least airflow to said first group to provide said requested engine output.
In this way, it is possible to operate with reduced numbers of cylinders carrying out combustion while, and thereby provide engine reduced engine output without passing the engine misfire limit, while at the same time maintain low emissions since excess oxygen does not dilute the combusted gasses fed to an exhaust system emission control device.
Note that, in one example, the emission control devices utilized are three way catalysts. However, other devices could be used. Further, additional emission control devices could be used. Note also that the first and second cylinder groups can have equal or unequal cylinder numbers and can have only one cylinder in the group.
Advantages of the above aspects of the present invention are a fuel economy improvement with reduced costs and a reduced NOx or CO/HC emissions impact. Further, improved drivability is obtained by reducing the drive feel associated with constraints imposed by the minimum misfire load limits of the engine.